Dresden 2011 – scientific programme

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O: Fachverband Oberflächenphysik

O 36: Poster Session II (Metals; Nanostructures at surfaces; Surface or interface magnetism; Spin-Orbit Interaction at Surfaces; Electron and spin dynamics; Surface dynamics; Methods; Theory and computation of electronic structure)

O 36.122: Poster

Tuesday, March 15, 2011, 18:30–22:00, P4

Electronic structure and optical spectrum of lonsdaleite — •Johannes Büngeler, Stephan Blankenburg, and Arno Schindlmayr — Department Physik, Universität Paderborn, 33095 Paderborn, Germany

Lonsdaleite is a rare but naturally occurring allotrope of carbon. Like in diamond, each carbon atom forms four sp³-hybridized bonds with a local tetrahedral geometry, but the overall crystal structure is hexagonal, not cubic. First discovered in fragments of the Canyon Diablo meteorite in 1967, it was originally characterized as translucent with a brownish-yellow color and a Mohs hardness of 7–8, whereas recent first-principles calculations predicted a substantially higher indentation strength than that of diamond. If confirmed, this would make lonsdaleite the hardest material known. To resolve the conflicting experimental and theoretical findings, we perform a quantitative study of the quasiparticle band structure, using the GW approximation for the nonlocal and dynamic self-energy. Subsequently, the optical absorption spectrum is calculated from time-dependent density-functional theory, including a long-range kernel to account for the electron-hole attraction and the formation of excitons. Our results show unambiguously that pure lonsdaleite crystals are completely transparent in the visible spectral range. This gives strong support to the hypothesis that the analyzed samples were contaminated by impurities and imperfections, which not only influence the color, but also reduce the hardness.